This invention relates generally to fiber optic grating systems and more particularly, to the measurement of environmental effects using fiber optic grating sensors. Typical fiber optic grating sensor systems are described in detail in U.S. Pat. Nos. 5,380,995, 5,402,231, 5,592,965, 5,841,131 and 6,144,026.
The need for low cost, high performance fiber optic grating environmental sensor system that is capable of long term environmental monitoring, virtually immune to electromagnetic interference and passive is critical for such applications as moisture sensing and monitoring of adhesive bonds. Another advantage of these systems is that when they are appropriately configured the frequency response of the system can be very high.
The present invention includes fiber grating sensors that may be configured in appropriate transducers to sense environmental changes such as moisture, pressure, strain and temperature by using appropriate transducers. Means are also described to greatly reduce temperature sensitivity while enhancing accuracy and sensitivity.
In U.S. Pat. Nos. 5,380,995 and 5,397,891 fiber grating demodulation systems are described that involve single element fiber gratings and using spectral filters to demodulate fiber gratings. The present invention includes means to extend the demodulation system to enhance sensitivity and reduce temperature dependence. In U.S. Pat. Nos. 5,591,965, 5,627,927 the usage of fiber gratings to detect more than one dimension of strain is described. These ideas are extended in U.S. Pat. Nos. 5,869,835, 5,828,059 and 5,841,131 to include fibers with different geometries that can be used to enhance sensitivity or simplify alignment procedures for enhanced sensitivity of multi-parameter fiber sensing. In U.S. Pat. No. 6,335,524, “High Speed Demodulation Systems for Fiber Grating Sensors”, by Eric Udd and Andreas Weisshaar means are described to process the output from multiaxis fiber grating sensors for improved sensitivity. All of these patents' teachings are background for the present invention which optimizes the fiber grating sensor for optimum response to strain changes with reduced temperature sensitivity induced by changes in the state of its transducer to form water/chemical sensors, pressure and strain sensors.
The present invention consists of an optical fiber whose axial and or temperature sensitivity has been optimized through the construction of the optical fiber or mechanical mechanisms to enhance sensitivity. Temperature compensation is provided by a second fiber grating filter that is placed for first order temperature correction.
Therefore, it is an object of the present invention to monitor changes in moisture or chemical content of an environment through measured strain changes.
Another object of the invention is to provide a means of monitoring strain with reduced temperature sensitivity.
Another object of the invention is to reduce low frequency mechanical vibration noise from high frequency mechanical vibration signals.
Another object of the invention is to measure temperature with high accuracy and repeatability.
These and other objects and advantages of the present invention will become apparent to those skilled in the art after considering the following detailed specification including the drawings wherein: